This invention relates to the control of an actuator of a fuel injector incorporating a shape memory alloy element.
Various metallic materials capable of exhibiting shape-memory characteristics are well known in the art. These shape-memory capabilities occur as the result of the metallic alloy undergoing a reversible crystalline phase transformation from one crystalline state to another crystalline state with a change in temperature and/or external stress. In particular, it was discovered that alloys of nickel and titanium exhibited these remarkable properties of being able to undergo energetic crystalline phase changes at ambient temperatures, thus giving them a shape-memory. These shape-memory alloy (xe2x80x9cSMAxe2x80x9d) materials, if deformed while cool, will revert, exerting considerable force, to their original, undeformed shape when warmed. These energetic phase transformation properties render articles made from these alloys highly useful in a variety of applications. An article made of alloy having shape-memory properties can be deformed at a low temperature from its original configuration, but the article xe2x80x9cremembersxe2x80x9d its original shape, and returns to that shape when heated.
For example, in nickel-titanium alloys possessing shape-memory characteristics, the alloy undergoes a reversible transformation from an austenitic state to a martensitic state with a change in temperature. This transformation is often referred to as a thermoelastic martensitic transformation.
The reversible transformation of the NiTi alloy between the austenite to the martensite phases occurs over two different temperature ranges which are characteristic of the specific alloy. As the alloy cools, it reaches a temperature (Ms) at which the martensite phase starts to form, and finishes the transformation at a still lower temperature (Mf). Upon reheating, it reaches a temperature (As) at which austenite begins to reform and then a temperature (Af) at which the change back to austenite is complete. In the martensitic state, the alloy can be easily deformed. When sufficient heat is applied to the deformed alloy, it reverts back to the austenitic state, and returns to its original configuration.
SMA materials previously have been produced in bulk form, in the shape of wires, rods, and plates, for utilities such as pipe couplings, electrical connectors, switches, and actuators, and the like. Actuators previously have been developed, incorporating shape-memory alloys or materials, which operate on the principal of deforming the shape-memory alloy while it is below its phase transformation temperature range and then heating it to above its transformation temperature range to recover all or part of the deformation, and, in the process of doing so, create movements of one or more mechanical elements. These actuators utilize one or more shape-memory elements produced in bulk form, and, therefore are limited in size and usefulness.
The unique properties of SMA""s further have been adapted to microelectromechanical systems (xe2x80x9cMEMSxe2x80x9d) applications such as micro-valves and micro-actuators by means of thin film technology. Micro-actuators are desirable for such utilities as opening and closing valves, activating switches, and generally providing motion for micro-mechanical devices. The most well-known and most readily available SMA is an alloy of nickel and titanium. NiTi SMA has been extensively investigated as one of the most promising materials for MEMS such as microvalves and microactuators. NiTi SMA features the major advantages of having a large output force per unit volume, and the capability to serve as structural components as well as active components. It is reported that the advantageous performance of micro-actuators is attributed to the fact that the shape-memory effect of the stress and strain can produce substantial work per unit of volume. For example, the work output of nickel-titanium shape-memory alloy is of the order of 1 joule per gram per cycle. A shape-memory film-actuator measuring one square millimeter and ten microns thick is estimated to exert about 64 microjoules of work per cycle. With a temperature change of as little as about 10xc2x0 C., this alloy can exert a pressure or stress of as much as 415 MPa when applied against a resistance to changing its shape from its deformation state.
The application of shape memory alloy materials to a precision metering device such as an automotive fuel injector presents additional significant challenges in terms of control of the temperature related shape transformation corresponding to the inherent bi-stable crystalline structure of the SMA element. While the high temperature state transformation of the SMA element can be quickly accomplished by application of relatively high power, transition of the SMA element back to its ambient temperature state is dependent on cooling of the SMA element by fluid circulating through the injector upon termination of power application to the SMA element.
Now, according to the present invention, an improved apparatus and method for opening and closing an SMA flow actuator in a fluid stream has been developed. The actuator apparatus includes a shape memory alloy element that positions a valve element to a closed or open position, and a flow plate situated in the fluid stream in a position upstream of the SMA element. The flow plate features a through-hole pattern designed to increase the velocity of fluid flowing around the SMA element while it is in its activated, high temperature state. Preferably, the flow plate additionally is positioned to serve as a heatsink element to prevent SMA element overheating during activation.
The operation of the actuator involves applying a voltage to the SMA element to transform it to an activated position in its elevated temperature state. To transform the SMA element to its deactivated position, the applied voltage is removed, and the flow of fluid through the flow plate quickly cools the SMA element causing it to return to its deactivated position. When the SMA element is in its activated position, preferably, it is moved into thermal contact with the flow plate which also serves as a heatsink element.